Sencha isn't even remotely cross-platform, with support restricted to the latest webkit-based browsers with decent CSS3 support. Sencha's UI is pretty bad on any other device but an iPhone.

It sounds like you're talking about Sencha Touch, which is very different from the desktop version (ExtJS).

Correct, I meant Sencha Touch.

Yes, it is very convenient to work with if you're used to programming desktop UIs. I found that ExtJS is great for (admin) backends especially now that it integrates charting.

... and SmartClient.